4 submains with one common CPC

[atm84]

I have a query. 4 distribution boards, each fed by 25mm2 double insulated tails (approx 30m away) in a roof void. Only 1 16mm2 earthing conductor for all 4 submains. The earthing conductor is then connected at the first board and looped to the other three boards. Is this acceptable? I thought BS 7671 states that each circuit should have its own CPC.

 

[Pete999]

I have a query. 4 distribution boards, each fed by 25mm2 double insulated tails (approx 30m away) in a roof void. Only 1 16mm2 earthing conductor for all 4 submains. The earthing conductor is then connected at the first board and looped to the other three boards. Is this acceptable? I thought BS 7671 states that each circuit should have its own CPC.

Not really a cpc is it? main earthing conductor is my take

 

[Deleted member 26818]

As far as I can recall, the Regulations allow for a single CPC for multiple circuits, as long as that CPC is suitable for the combined expected Earth Fault Loads.
The 4 DBs, are they supplied by separate tails from a single point, or are the tails daisy-chained from one board to another?

 

[davesparks]

I have a query. 4 distribution boards, each fed by 25mm2 double insulated tails (approx 30m away) in a roof void. Only 1 16mm2 earthing conductor for all 4 submains. The earthing conductor is then connected at the first board and looped to the other three boards. Is this acceptable? I thought BS 7671 states that each circuit should have its own CPC.

Do you know where in bs7671 you think it states this?
It would be best to check your copy of the regs rather than going on what you think it says.

 

[atm84]

Not really a cpc is it? main earthing conductor is my take

Yes very true. I guess it's a CPC in regards to it being a sub main circuit but then it's also a main earthing conductor for the DB.

 

[atm84]

As far as I can recall, the Regulations allow for a single CPC for multiple circuits, as long as that CPC is suitable for the combined expected Earth Fault Loads.
The 4 DBs, are they supplied by separate tails from a single point, or are the tails daisy-chained from one board to another?

The 4 DBs are supplied by separate tails. So there are live and neutral for each DB (8 tails in the loft in total) and 1 CPC/ earthing conductor.

 

[atm84]

Do you know where in bs7671 you think it states this?
It would be best to check your copy of the regs rather than going on what you think it says.

Yes I found the reg number today. I don't have a copy to hand but will forward the reg number when back in the office. What are your thoughts to the OP. What would you say If you came across the situation and were asked to add additional circuits from the DBs in question.

 

[jedisparks]

What is protecting the tails? Does each set of tails go back to an individual switch fuse for example?

 

[davesparks]

Yes I found the reg number today. I don't have a copy to hand but will forward the reg number when back in the office. What are your thoughts to the OP. What would you say If you came across the situation and were asked to add additional circuits from the DBs in question.

Of you have already found the regulation which allows this then why do you need to ask?

I wouldn't say anything at all as long as the cpc is correct for the most onerous possible fault.

 

[davesparks]

What is protecting the tails? Does each set of tails go back to an individual switch fuse for example?

Does it matter?

 

[atm84]

Yes a separate switch fuse. The CPC/ earthing conductor that serves the 4 DBs originates from the MET

 

[atm84]

Of you have already found the regulation which allows this then why do you need to ask?

I wouldn't say anything at all as long as the cpc is correct for the most onerous possible fault.

Even though I found a reg that states each circuit should have its own CPC, something tells me it's not right. I'm asking for other ppls opinion. Like has already been mentioned, it's also an earthing conductor as well as a CPC.

 

[atm84]

Of you have already found the regulation which allows this then why do you need to ask?

I wouldn't say anything at all as long as the cpc is correct for the most onerous possible fault.

Davesparks rather than trying to find fault with my post and being unhelpful why don't you say what your opinion is and put your neck out there instead of hiding behind screen. What is your opinion? What would you do if you keep across the situation. I've said what I think but like I said I'm not confident and am asking for other people's opinion.

 

[jedisparks]

Was just trying to establish if this is actually a cpc or main earthing conductor we're talking about. But as you've said above if it satisfies most onerous condition it doesn't matter. If it's suitably sized etc.

Cheers

 

[davesparks]

Even though I found a reg that states each circuit should have its own CPC, something tells me it's not right. I'm asking for other ppls opinion. Like has already been mentioned, it's also an earthing conductor as well as a CPC.

Which regulation number is it? I've never heard of it and it directly opposes the regulation which allows this situation.

Such a regulation would render any steel conduit or trunking installation non-compliant